To dream that you (or your romantic partner) has a mistress may suggest your unconscious desire to end your current relationship. You may be finding ways to sabotage it. Alternatively, you could feel neglected in the relationship.
If you are single and have this dream, it suggests that you or someone in your circle is being lured or tempted into some negative activity.
To dream that you are someone’s mistress indicates your desire to get things without working for them.... My Dream Interpretation